1. Field of the Invention
The present invention relates to a radio communication method and a radio communication system, and particularly to a radio communication method and a radio communication system suitable for use in the case where in a radio communication system such as a cellular system, traffic concentrates on a specific place in a multisector model wherein one cell is operated as a plurality of sectors.
2. Description of the Related Art
Attention has recently been focused on CDMA (Code Division Multiple Access) as a communication method suitable for a mobile radio or wireless terminal such as a cellular phone. A multisector model of a type wherein a cell is divided into a plurality of sectors and operated in this condition, has been proposed as a communication model for the CDMA.
A description will first be made of a communication model for a radio communication system according to a related art with reference to FIG. 11.
FIG. 11 is a conception diagram showing a configuration of the communication model for the radio communication system according to the related art.
A base station 11 is connected to a base station controller 13 through a communication line 12. The base station controller 13 is connected to an exchange or switching system 14. Further, the base station controller 13 is connected to one or plural base stations 11 through the communication line 12 and has the function of performing switching between signals sent from the termination of a radio interface and each base station 11, switching between channels, etc.
In the present example as well, one cell operated by the base station 11 is divided into three sectors S1 through S3 by directivity antennas. In such a conventional CDMA cellular system, the sectors S1 through S3 constituting the cell are transmitted in the form of uniform transmitting power distributions at in-sector angles. Traffic for entire sector has been taken into consideration at this time but a traffic distribution inside each sector is not taken into consideration. Further, a Rake receiver has been known as a CDMA diversity technique. The Rake receiver has been described in the following:
R. D. Blakeney, et al, xe2x80x9cDemodulation Element Assignment in a System Capable of Receiving Multiple Signalsxe2x80x9d, U.S. Pat. No. 5,490,165 Feb. 6, 1996, and the like.
In radio communications, transmitting power also commonly increases with an increase in traffic. In the case of actual traffic, however, traffic-concentrated locations and relatively traffic non-concentrated locations exist. In the example illustrated in FIG. 11, for instance, areas or the like around stations A and B are considered to be traffic-concentrated locations.
Therefore, the base station 11 should inevitably increase transmitting power. However, a uniform increase in transmitting power with respect to a cell results in the use of much transmitting power even at a low traffic portion, thus causing waste. Since the transmitting power can be changed every sectors in the case of such a multisector that one cell is divided into the sectors as shown in FIG. 11 as compared with it, the wastage of the transmitting power can be limited to some extent. In order to restrict the transmitting power in total, the sector may be divided into pieces with respect to an area in which traffic is expected to concentrate. However, when the sector is excessively divided into pieces, channels must be prepared for communications at their boundaries, thus causing a problem on a so-called handoff margin that the number of effective channels decreases, and a problem that handoff often occurs at terminals of users who moves at high speed. Interference between sectors also offers a problem. It is therefore hard to ensure the quality of communication.
The present invention has been made to solve the above-described problems. The present invention provides a radio communication method and a radio communication system wherein even if traffic concentrates on a specific place in a cellular radio communication in which a base station communicates with its corresponding wireless terminal, power to be used up or consumed by the base station can be restricted, and the equality of communication is good and the need for a change in the conventional wireless terminal is eliminated.
According to a first aspect of the invention, there is provided a cellular radio communication method for performing communications between a base station and each of wireless terminals, comprising the steps of dividing a cell for covering the base station into several sectors, defining the sectors as a wide range sector and two types of narrow range sectors which overlap with the wide range sector, separately preparing a wide range antenna for covering the wide range sector and narrow range antennas for respectively covering the narrow range sectors within the base station, and transmitting signals to wireless terminals located within the narrow range sectors by using both the wide range antenna and the narrow range antennas.
In the radio communication method, the signal transmitted through the wide range antenna and the signals transmitted through the narrow range antennas are set so as to become the same signal.
In the radio communication method, when a plurality of narrow range sectors exist within one wide range sector, respective coverages for the plurality of narrow range sectors do not overlap in position.
In the radio communication method, the base station receives signals sent from the wireless terminals by the wide range antenna and one narrow range antenna and combines both received signals together to thereby extract a combined signal.
According to a second aspect of the invention, there is provided the radio communication method wherein the communications between the base station and the wireless terminals are based on CDMA (Code Division Multiple Access).
In the radio communication method, the base station transmits only a signal on a traffic channel within channels handled by CDMA by using both the wide range antenna and one narrow range antenna.
In the radio communication method, the base station extracts downlink power control information included in the signals sent from the wireless terminals and controls transmitting power directed toward each wireless terminal according to the downlink power control information upon transmission of the signals through the wide range antenna and the one narrow antenna.
In the radio communication method, the base station controls a transmitting power ratio directed toward each wireless terminal according to the ratio between the strengths of the signals from the wireless terminals, which are received by the wide range antenna and the narrow range antennas upon transmitting the signals through the wide range antenna and the one narrow range antenna.
In the radio communication method, the transmitting power ratio directed toward the wireless terminal is determined in advance by the base station upon transmitting the signals through the wide range antenna and the narrow range antennas.
In the radio communication method, both the base station and the wireless terminal Rake-combine received signals upon combination thereof.
According to a third aspect of the invention, there is provided a cellular radio communication system for performing communications between a base station and each of wireless terminals, comprising a cell for covering the base station, the cell being divided into several sectors which are defined as two types of sectors corresponding to a wide range sector and narrow range sectors overlapping with the wide range sector, and the narrow range sectors having coverages provided so as not to overlap in position, and wherein the base station includes a wide range antenna for covering the wide range sector, narrow range antennas for respectively covering the narrow range sectors, a transmitter for transmitting signals to wireless terminals located within the narrow range sectors by using the wide range antenna and the narrow range antennas, a receiver for combining a signal received by the wide range antenna and signals received by the narrow range antennas together to thereby extract a combined signal, and a controller for controlling the transmitter and the receiver, and the controller controls the transmitter so that the transmitter transmits signals to the wireless terminals located within the narrow range sectors through both the wide range antenna and the one narrow range antenna.
In the radio communication system, the communications between the base station and each wireless terminal are based on CDMA, and the controller in the base station controls the transmitter so that the transmitter transmits only signals on traffic channels as the same signal from both the wide range antenna and the one narrow range antenna.
In the radio communication system, the controller in the base station controls a transmitting power ratio directed toward each wireless terminal according to the ratio between the strengths of the signals from the wireless terminals, which are received by the wide range antenna and the narrow range antennas upon transmitting the signals through the wide range antenna and the one narrow range antenna.
In the radio communication system, the wide range antenna and each narrow range antenna of the base station transmit polarized signals different from one another, and each wireless terminal has an antenna for receiving both polarized signals and a receiver for combining the received polarized signals together to thereby take out a combined signal.
According to a fourth aspect of the invention, there is provided a base station suitable for use in a cellular radio communication system for carrying out communications between the base station and each of wireless terminals, wherein a cell for covering the base station is divided into several sectors, which are defined as two types of sectors corresponding to a wide range sector and narrow range sectors overlapping with the wide range sector, and the narrow range sectors have coverages provided so as not to overlap in position, and wherein the base station includes a wide range antenna for covering the wide range sector, narrow range antennas for respectively covering the narrow range sectors, a transmitter for transmitting signals to wireless terminals located within the narrow range sectors by using the wide range antenna and the narrow range antennas, a receiver for combining a signal received by the wide range antenna and signals received by the narrow range antennas together to thereby extract a combined signal, and a controller for controlling the transmitter and the receiver, and the controller controls the transmitter so that the transmitter transmits signals to the wireless terminals located within the narrow range sectors through both the wide range antenna and the one narrow range antenna.
Typical ones of various inventions of the present inventions have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description.